To manufacture an elongated rubber product, an extrusion molding method has been used. The extrusion molding method is a molding method in which a molding material such as rubber, PP (polypropyrene) and PVC (polyvinyl chloride) is molded into a desired configuration by extruding the molding material from an extrusion nozzle (die) having an opening corresponding to the configuration of a molded article under pressure while kneading and plasticizing the molding material with a screw.
When the article has a uniform cross-sectional configuration, the molding material may be continuously extruded from an invariant die having a definitely configured opening. However, in manufacturing the extrusion molded article of which the cross-sectional configuration is longitudinally modified, the configuration and the area of the die opening must be changed to conform to the cross-sectional configuration of the molded article.
An example of such an extrusion molded article is a weather strip for an automobile body shown in FIG. 9(A), FIG. 9(B), FIG. 9(C) and FIG. 10.
As shown in FIG. 9(A), the weather strip 50 is mounted along a circumferential edge of a windshield 42 of the automobile body 40. The weather strip 50 includes side sections 52 extending along both sides of the windshield 42 and an upper section 54 extending along an upper edge of the windshield 42 and is integrally molded by extrusion molding, as shown in FIG. 10.
As shown in a sectional view of FIG. 9(B), the side section 52 has a cross-sectional configuration centrally provided with a partition wall 52a to prevent flowing of rainwater toward side surfaces of the automobile body 40. On the other hand, as shown in FIG. 9(C), the upper section 54 does not include such a partition wall and has a cross-sectional configuration smaller than that of the side section 52. Therefore, in extrusion molding of the weather strip 50, the cross-sectional configuration must be changed during a single molding process (1 cycle) for one piece of the molded article, as shown in FIG. 10.
To this end, as shown in FIG. 11, a movable die 208 is provided on a die 204 of an extrusion molding machine 201, which may change the configuration of a die opening 210. The movable die 208, although not illustrated in detail in FIG. 11, is constituted of two movable die halves which are movable in a direction crossing at right angles relative to each other. One of the movable die halves, when positioned, defines the top-to-bottom dimension in FIGS. 9(B) and 9(C). The other of the movable die halves, when positioned, defines the configuration of the right-to-left constricted portion in FIGS. 9(B) and 9(C).
The construction of the extrusion molding machine including such two movable die halves is identical with that described in previous patent applications, for example, Japanese Laid-Open Patent Publication No. 4-356216.
The position of the movable die 208 is moved by a control mechanism (not shown) in synchronous with an extruding operation of an extrusion screw S so that the configuration of the die opening 210 is changed between the cross-sectional configuration 52 in FIG. 9(B) and the cross-sectional configuration 54 in FIG. 9(C).
Thus, as shown in FIG. 10, the weather strip 50 is formed in which the cross-sectional configuration thereof is modified for a desired length.
Now, in manufacturing an extrusion molded article such as the weather strip 50 of which the cross-sectional configuration is modified, it is important to constantly form the molded article which has no variation in quality and which has a desired dimension of cross-sectional configuration at each section thereof. For this purpose, the molding material has to be controlled to be stably extruded in proper amounts for each section having a different cross-sectional configuration.
However, in the extrusion molding of such an article, the area of the die opening 210 changes since the movable die 208 is moved to conform to the cross-sectional configuration of the molded article. This may vary of inner pressures of the fixed die (hereinafter referred to as "back pressures") applied to the molding material to be extruded, and amounts of the molding material extruded from the die opening 210 and the like.
Such variations in the back pressures and the amounts of the molding material to be extruded may cause variations in the magnitude of expansion of the molding material (die swelling) which may be caused when it is released from the back pressures upon being extruded from the die opening, and may in turn lead to errors of the cross-sectional dimensions of the article.
Then, in order to preclude the variations in the back pressures and the amounts of the molding material to be extruded for manufacturing the extrusion molded article having desired cross-sectional dimensions, many kinds of extrusion molding methods have been developed.
Such extrusion molding methods are described in, for example, Japanese Laid-Open Patent Publication No. 5-104604 entitled "Method and Apparatus for Manufacturing Weather Strip" and Japanese Laid-Open Patent Publication No. 5-104605 entitled "Method for Manufacturing Weather Strip".
The technique described in the former Publication relates to a method to detect the variations in the back pressures adjacent to the extrusion die due to changes in the area of the die opening for controlling an opening degree of back pressure control bore. In this method, when the back pressures are increased upon reduction of the area of the die opening, a control is carried out so that an opening degree of the back pressure control bore is increased for releasing an excess of molding material.
As a result, a suitable amount of molding material is extruded from the die opening, and the back pressures applied on the molded body may also fall within a suitable back pressure range. This may prevent the dimensional errors in the extrusion molded article due to variations in the die swelling.
The technique described in the latter Publication relates to a method to detect the variations in the back pressures adjacent to the extrusion die due to changes of the area of the die opening for controlling revolutions of an extrusion screw based on values of the back pressures thereby controlling extrusion pressures of the molding material.
In this method, the screw revolutions are not necessarily controlled to keep the back pressures constant, and are controlled in such a manner that the back pressures correspond to desired values best suited to the cross-sectional configuration at each section of the molded article.
As a result, undesirable variations in the back pressures are prevented and variations in the cross-sectional dimensions of the molded article due to the die swelling are restricted, so that the cross-sectional dimensions of the molded article are controlled.
However, the method to control the back pressure by releasing the molding material through the back pressure control bore wastes the molding material released therethrough and increases production costs of the article.
Further, in the method in which the screw revolutions are feedback controlled on the basis of the variations in the back pressures, the following problems arise because of time delays as shown in FIGS. 12(A) and 12(B).
FIG. 12(A) is a chart showing the changes of the back pressures adjacent to the die opening 210 and head pressures in the vicinity of the forward end of the extrusion screw S upon operation of the movable die 208 shown in FIG. 11. Alternatively, FIG. 12(B) is a chart showing the changes of the back pressures and the head pressures at the time when the revolutions off the extrusion screw S are changed.
Further, the back pressures and the head pressures are determined by a pressure sensor 206 positioned adjacent to the die opening 210 of an extrusion molding machine 201 and a pressure sensor 228 positioned adjacent to the forward end of the extrusion screw S, respectively.
As shown in FIG. 12(A), as the movable die 208 is moved to increase the area 60 of the die opening 210, the back pressures 62 adjacent to the die opening 210 are immediately decreased, whereas the head pressures 64 are decreased with a time delay T.sub.51.
This is because a considerable time is required to propagate changes of pressures from the area adjacent to the die opening 210 that creates the back pressure to the extrusion screw S because of the distance therebetween. Similarly, as the movable die 208 is moved to decrease the area 60 of the die opening, the head pressures 64 are increased with a time delay T.sub.52.
Likewise, a considerable time is required to propagate changes of pressures from the forward end of the extrusion screw S to the die opening 210. Therefore, as shown in FIG. 12(B), the back pressures 62 are changed with time delays T.sub.53 and T.sub.54 where the screw revolutions 66 are changed.
For the reasons above, in the conventional method in which the screw revolutions are controlled by the feedback controlling, a problem as shown in FIG. 13 arises. Further, the control for the extrusion molding shown in FIG. 13 is an exemplary method in which it is intended to keep the back pressures constant for obtaining a good molded article.
In this control, there exist time delays T.sub.55 and T.sub.56 since changes 68 of the back pressures 62 due to the changes of the area 60 of the die opening are determined at a point adjacent to the die opening before controlling the screw revolutions 66. The time delays T.sub.55 and T.sub.56 may produce variations 70 and 72 of the back pressures. Consequently, the back pressures 62 can not be maintained constant. Such variations 70 and 72 of the back pressures have caused a problem of producing the errors of the cross-sectional dimensions or variations in quality of the molded product.